The present invention relates to the structure and performance of materials with useful thermoelectric characteristics at temperatures less than 200° C., and the production thereof by less expensive bulk material processing techniques.
Thermoelectric materials exhibit a predictable relationship between their electrical and thermal performance. Depending on the desired outcome, a thermoelectric apparatus is frequently arranged as either a cooling/heat pump device or as an electrical power generator. To use the material as a cooling device or heat pump, an electric field is applied across the material, typically via an electric circuit. This field generates a thermal gradient according to the Peltier effect so long as it is maintained. Heat can be removed from the hot side of the material by a heat sink, heat exchanger or other heat removing means. When both are done simultaneously, the apparatus then operates effectively as a cooling device or heat pump. Conversely, creating a thermal gradient across the material, such as by application of thermal energy, induces an electric field according to the Seebeck effect. Connecting the material to a circuit aligned with this field causes the apparatus to operate effectively as an electrical power generator.
There are numerous benefits of applying thermoelectric materials to cooling and electrical power generation. The total size of the thermoelectric materials and ancillary parts of a thermoelectric cooling apparatus can be relatively small compared to the size of an equivalent cooling system with typical components. Most typical cooling systems in current production utilize a vapor compression system, which requires at least a compressor, a working fluid, an expansion valve, an evaporator, and a condenser. Likewise, typical power generation facilities utilize large steam turbines to convert heat energy into electricity. A thermoelectric apparatus is smaller because there is less need to house this large, expensive equipment to convert energy from one form to another. This reduces the amount of space necessary to operate a cooling or power generation system, saving valuable functional space for a manufacturer, designer, or builder. Therefore, the compact size of thermoelectric materials the makes them ideal for many cooling and power generation applications where space is at a premium.
End users also benefit from the virtual maintenance-free operation of a thermoelectric apparatus. Typical systems as described above use mechanical intermediaries to convert energy between a thermal form and an electrical form, while thermoelectric materials directly convert thermal energy to electrical energy without the need for mechanical intermediaries. Moving parts such as turbines, motors, and pumps, even if reliable, typically require periodic inspection and maintenance to minimize unplanned outages and major repairs and tend to reduce overall efficiencies. A thermoelectric apparatus has fewer such ancillary parts, and therefore fewer opportunities for system downtime.
Despite the size and maintenance advantages over current systems, thermoelectric materials have to date been limited to certain niche and specialty applications. Key factors limiting widespread adoption of the technology are high production costs combined with the practical size requirements for thin film materials. The achievable thermoelectric performance of current materials, especially those designed to operate near room temperature, is also inadequate to make thermoelectric cooling systems competitive for many large scale operations, further restricting broad adoption of current thermoelectric technology.
Current thermoelectric materials are best suited for applications with virtually no cost restrictions, that have a significant size constraint, or where reliability is more critical than efficiency. For example, current power generation applications include those in remote unattended land-based or space-based operations. Cooling technology using thermoelectric materials is generally limited to small-scale cooling applications, particularly in the medical and scientific fields. Thermoelectric cooling is penetrating the consumer market in certain specialty products such as portable refrigeration units and heated/cooled automotive seats. Each of these applications have particular reliability or size requirements that make thermoelectric cooling more attractive than standard vapor compression systems. In these applications, specialized needs outweigh the limited performance available from current thermoelectric materials.
The performance of a thermoelectric material is characterized by its dimensionless figure-of-merit, ZT:
                    ZT        ≡                                            S              2                                      ρ              ⁢                                                          ⁢              k                                ⁢                      T            M                                              [        1        ]            where ρ is the electrical resistivity, S is the Seebeck coefficient or thermoelectric power, TM is the average of the hot and cold side temperatures, and k is the thermal conductivity. In this equation, thermal conductivity, k, is the sum of an electrical contribution, kel and a phonon contribution kph, also expressed as k=kel+kph.
As ZT increases, so does the conversion efficiency. While it is desirable to increase ZT by increasing S or decreasing ρ or k, there has been limited progress in the ability to beneficially change only one factor without causing a counteracting change in the other. For example, kel and ρ are related by the Wiedemann-Franz law, so that a decrease in ρ typically causes an offsetting increase in kel, resulting in no significant improvement. A reduction in kel can similarly cause an offsetting increase in ρ. These offsetting changes in kel and ρ typically result in no significant improvement in ZT. Most advances in thermoelectric performance have come from fabricating materials with reduced kph.
However, selective reduction in kph has generally been possible only with labor and capital intensive thin film processes such as chemical vapor deposition. These processes reduce kph by creating successive phonon blocking layers or inclusions in one or two dimensions by building the materials several atoms at a time, but not in bulk. These current fabrication processes and their associated costs limit the size, performance, and application of thermoelectric materials.
Not only do thin film products have a high production cost per unit volume, there are also practical size limits on materials produced via this process. Limits on thin film thermoelectric materials begin to arise in materials with thickness less than about 30 microns. At that level, parasitic losses begin to degrade overall device performance compared to the performance at the material level. Therefore, while vapor-deposited materials have relatively high ZT at room temperature, larger dimensioned thermoelectric materials are prohibitively expensive and insufficient for all but the most cost-insensitive applications.